1. Field of the Invention
The disclosed technology relates to the fields of hybrid assembly of microsystems, parallel stochastic assembly and 3D electronics integration.
2. Description of the Related Technology
Flip-chip is presently the most common technique used to build up heterogeneous electronic systems. It includes mechanically picking up the individual components (i.e. devices, in the following equivalently referred as chips or parts) of the system, moving them to their final location in the system (e.g. on a common planar substrate), and finally mechanically and electrically binding them in place.
In the perspective of device downscaling and improved assembly throughput, such technique presents some problems: it is serial; it requires closed-loop external control and direct contact of the handling tool with the involved parts.
Fluid-mediated parallel stochastic assembly (also known as capillary-driven self-assembly) is a promising alternative to flip-chip which potentially overcomes its issued shortcomings. In its essence, it exploits the substrate as a template to organize the parts and to drive them into correct placement. The assembly takes place in a liquid environment, which allows avoiding direct handling of the individual parts, while they can be processed in parallel.
In A. K. Verma, M. A. Hadley, H.-J. J. Yeh, J. S. Smith, “Fluidic self-assembly of silicon microstructures,” Proceedings of 45th Electronic Components and Technology Conference, (1995), 1263-1268., cavities etched into the planar substrate are exploited, whose tri-dimensional geometry allows for chips of complementary shapes to match and selectively fit therein (driven by gravity) as they are spread over the substrate in a fluid medium. In S. A. Stauth and B. A. Parviz, “Self-assembled single-crystal silicon circuits on plastic”, PNAS vol. 103, p. 13922-13927 (2006), the use of the grooves, which are realized in a thick resist layer above the substrate, for part placement is combined with the use of molten-solder capillary forces for their binding.
In U. Srinivasan, D. Liepmann and R. T. Howe, “Microstructure to substrate self-assembly using capillary forces”, IEEE JMEMS vol. 10, p. 17-21 (2001), and K. L. Scott et al., “High-performance inductors using capillary based self-assembly”, IEEE JMEMS vol. 13, p. 300-309 (2004), two-dimensional shape matching and capillary forces for chip self-alignment are used. A hydrophilic planar substrate is lithographically patterned with purposely-designed binding sites which are treated to be hydrophobic (e.g. by applying a self-assembling monolayer). The chips to be assembled are also similarly treated so to make their functional side (i.e. the one exposing the electrical connections) hydrophobic while leaving the remaining sides hydrophilic. During the assembly process, the binding sites on the substrate are selectively coated with a thin hydrophobic liquid film and subsequently submerged in a hosting aqueous fluid; this is required since the interface between the hydrophobic film and the hosting fluid has to be highly energetic. The chips to be assembled are then directed toward the substrate. Upon contact between the hydrophobic film on the binding sites and the hydrophobic side of the chips, the (interfacial) energy of the system is lowered, and this energy minimization (capillarity) induces a force on the chips which drives them into alignment with the underlying binding sites. The alignment performance and accuracy achievable depends on several parameters, mainly on the design of the sites and the chips, on the energy of the fluid interface and on the volume of the hydrophobic film deposited on the sites.
In fluidic assembly techniques according to the prior art, use is made of the interfacial energy between the aqueous hosting fluid (typically, water) and the hydrophobic fluid (typically, a hydrocarbon oil or polymer) to drive the assembly of chips. On the other hand, the chips themselves have their functional side made hydrophobic, and they are also immersed in the hosting fluid at the time of assembly.
Depending on the way the chips are delivered toward the substrate and on the way they move across it to search for the binding sites, the chance that they get into close vicinity is high. Hydrophobic surfaces immersed in aqueous fluids experience the so-called hydrophobic interaction, which tends to draw the surfaces in close contact. It is found that this has the effect of getting the chips to stick to each other. Such mutual binding is most likely pair-wise, and involves the functional side of the chips, which is thus hidden from exposure. The interaction force is relatively high; practically, once the chips are stuck they can no longer be detached, unless immersed in a fluid of lower surface tension [illustrated in FIG. 1] (but using such fluid for the assembly would also decrease the force driving the assembly itself). As a consequence, they can no longer be used for assembly purposes. Moreover, bubbles that may accidentally be produced in the hosting fluid may also easily interact with the hydrophobic surface of the chips: in this eventuality, aggregates involving more than two chips may form. It constitutes an undesired effect in these 2D assembly processes. The main consequence of such interference is evidently a reduction in process yield: chips that get stuck during delivery cannot be used for assembly, nor can they be recycled to be used in subsequent assembly steps. Equivalently, since a certain fraction of the chips may get involved in mutual adhesion, the quantity of chips required to achieve the complete assembly over the substrate needs to be larger than ideal, and this makes the process less efficient. The process is also sensitive to bubble formation.